The present invention relates to a method and an apparatus for welding workpieces having different heat capacities, for example, a cladding tube and a plug of a fuel rod used in a nuclear reactor, in tungsten inert gas (TIG) welding process.
Referring to FIG. 10 showing a fuel rod, fuel pellets 2 are inserted into a cladding tube 1 made of zircaloy. A spring 4 is also inserted in a plenum 3 inside the cladding tube 1. The plugs 5 and 6 are welded at both ends of the cladding tube 1, so that the fuel pellets 2 are urged to the plug 5.
The plugs 5 and 6 are welded to the cladding tube 1 of the fuel rod in a TIG welding process, which is carried out with a welding apparatus A as shown in FIG. 11.
In the TIG welding process, the plugs 5 and 6 are fitted into the both ends of the cladding tube 4 which includes the fuel pellets 2 and the spring 4. The cladding tube 4 is passed through a rotation holding means 7, and one end of the cladding tube 4 is further inserted through a hole 9 into a welding chamber 8 until the plug 5 or 6 comes into contact with a hollow 11 of a stopper 10, which is rotatably mounted to the opposing wall of the welding chamber 8.
The portion (surface) S where the cladding tube 1 and the plug 5 (or 6) mutually contact is welded with a welding apparatus 12 inserted from the upper side of the welding chamber 8, while the cladding tube 1 is being rotated by the rotation holding means 7.
Referring to FIG. 12, the welding apparatus 12 includes a welding electrode 14 disposed at the center of an opening of a gas nozzle 13. In use, the tip of the welding electrode 14 and the center of the gas nozzle 13 are placed adjacent to the contact portion (boundary portion) S. A current is passed between the welding electrode 14 and the contact portion S to form an arc discharge, which melts and welds the contact portion S. During the process, shield gas (inert gas) is introduced from the gas nozzle 13 and flows as illustrated with arrows in FIG. 11, preventing oxidation of the contact portion S.
In welding as described above, however, the cladding tube 1 which is one of the workpieces at the contact portion S, is cylindrical and hollow, while the plugs 5 and 6 are solid. The plugs 5 and 6 have therefore greater heat capacities, and there is a difference in temperature between the workpieces. The melting quantities differ between the cladding tube 1 and the plugs 5 and 6 at the contact portion S, and this leads to shift of the welding bead to the side of the cladding tube 1. The welding result is therefore unsatisfactory.
To avoid this, the arc is directed, in general, particularly toward the workpiece with the higher heat capacity. However, the arc spreads to the side of the workpiece with the lower heat capacity which is apt to be heated, resulting in inferior quality. Also, there is a problem in that an undercut U, as shown in FIG. 12, may occur on the cladding tube 1 with the low heat capacity.